Since the introduction of lead-acid batteries and electric starting motors to motor vehicles, other electrical loads in the motor vehicle have been able to discharge the battery to a level at which it can no longer start the vehicle engine. Various alarms (e.g. a buzzer that goes off when lights are left on and the driver's door is opened) have been added to warn the operator when these electrical loads are on and might over-discharge the battery. These alarms are critical for the headlights of motor vehicles because headlights are normally on a separate circuit, for safety reasons. The general object of the present invention is to prevent over-discharging of the vehicle battery, by various accessory loads, to this critical level.
In marine applications, a battery can be used to start the outboard motor (or other engine type) as well as providing power for such loads as a trolling motor and other accessories. As a general rule these loads have battery power applied to them at all times, even when the outboard motor is not running. A trolling motor can draw as much as 50 A of current. When the outboard motor is not running, any of these loads can discharge the battery to a state-of-charge (SOC) level that makes starting the engine difficult or impossible. This can be a nuisance when a fisherman (i.e. operator) is on an inland lake, and can be life threatening when the operator is on coastal waters, in a remote area of an inland lake or during extreme weather conditions.
Over-discharging a battery to the point where the boat engine can not be started is so prevalent in marine applications that few fishermen run the risk of sharing one battery for engine “starting” and for accessories. Therefore, a typical fishing boat has one battery for starting the engine and powering accessories and another battery (or batteries) for powering the trolling motor. Then, at least a 2-bank charger is required to maintain the batteries. Although somewhat less likely to occur, this still leaves the operator exposed to over-discharging the engine battery by the remaining accessories connected to it. A disadvantage of this solution is that it adds the additional weight and cost of an extra battery and a more complex charger. Another disadvantage is that the trolling motor battery(s) does not get charged when the boat engine is running, unless a specialized charger is also added to the electrical system.
Many in-vehicle attempts have been made to monitor the SOC of a vehicle battery and switch some or all of the battery loads off at an SOC level that allows effective starting of the vehicle engine. Examples are Von Brimer (U.S. Pat. Nos. 3,395,288 and 3,646,354), Russell (U.S. Pat. No. 4,039,903), Abert (U.S. Pat. No. 4,080,560), Sheldrake (U.S. Pat. No. 4,493,001), Sloan (U.S. Pat. Nos. 4,902,956 and 5,089,762), Gayler (U.S. Pat. No. 5,136,230), Morland (U.S. Pat. No. 5,140,250), Betton et al. (U.S. Pat. No. 5,200,877), Clokie (U.S. Pat. No. 5,272,380), Meister (U.S. Pat. No. 5,321,389) and Parsonage (U.S. Pat. Nos. 6,037,749 and 6,242,891 B1). Prior solutions have not been practical solutions because of their complexity, lack of overload protection, lack of reverse battery protection, excessive quiescent current consumption and/or inability to accurately determine the SOC of the vehicle battery. None of these prior solutions address the marine environment and, more specifically, the unique case of having a high current accessory load such as an electric trolling motor that is always operated when the boat engine is not running.
There are many factors that make it difficult to determine the SOC of a vehicle battery. First, battery voltage and battery current must be monitored at all times for all load conditions. Battery current must be monitored because a lead-acid battery has a significant internal resistance that causes the battery voltage to decrease with increasing load current. This internal resistance increases as the level of the SOC of the battery decreases. I measured a 23 mΩ internal resistance at a 40 A load current and approximately 20% SOC on a deep-cycle battery with a marine cranking amp (MCA) rating of 875 A. Another deep-cycle battery with an MCA rating of 840 A, measured a slightly higher internal resistance and a starting battery with a MCA rating of 1000 A, measured slightly lower. Therefore, the MCA or the cranking amp (CA) rating on a battery is inversely proportional to, yet an indirect measure of a battery's internal resistance. The internal resistance of a battery is non-linear with respect to load current. Internal resistance of the 875 A MCA rated battery (mentioned above) measured an internal resistance of 26 mΩ at 20 A decreasing to 21 mΩ at 80 A. At lower currents the internal resistance increased more rapidly, 30 mΩ at 10 A, 36 mΩ at 5 A, 40 mΩ at 2.5 A and 65 mΩ at 1.25 A. The error in sensed battery voltage caused by this changing internal resistance is typically <0.12V (or <5% SOC). Second, a lead-acid battery's voltage recovery from having its load switched between two levels is very “sluggish,” due to internal, chemical reactions. My own observation of several lead-acid batteries and several lead-acid battery types (e.g. starting, deep-cycle, different MCA ratings, etc.) has shown that they all exhibit this “sluggish” characteristic. When the load is suddenly increased on a battery, it can take 30 seconds or more for the internal resistance of the battery to decrease to within 10% of its final value. Also, when the load is suddenly decreased on a battery from a high current (say 60 A) to no load (0 A), it can take 10 minutes or more for the battery voltage to increase to within 0.1V of its final value. But, when the load is switched from a high current (say 60 A) to a low current (say 2.5 A), the time required to settle is typically reduced to <60 seconds. Recovery time is also typically <60 seconds when the battery load is switched from a low current (say 2.5 A) to no load (0 A). Therefore, timers are required to make an accurate determination of a battery's SOC. These same timers can prevent “cranking” current, to the vehicle engine, from causing a false detection of low SOC of the battery. Third, a lead-acid battery's energy storage capacity decreases with decreasing temperature. Other desirable features that a battery supervisor should incorporate are: minimum operator intervention, minimum interconnection to vehicle electrical wiring, low quiescent current, over-current protection, high surge current capability, over-temperature protection, ESD protection and sometimes, more than one switch.
As a lead-acid battery is discharged, it forms lead sulfate on the positive plates. If the battery is left in a discharged (or partially discharged) state for an extended period of time (even as little as 12 hours) the lead sulfate will begin to harden (i.e. crystallize). When this happens, the battery loses capacity and its internal resistance increases. Therefore, especially in marine applications, an on-board marine battery charger needs to be a part of the electrical system.
A switch (or switches) used to connect to and disconnect from vehicle loads must be able to deliver high surge currents. For example, an incandescent lamp can draw as much as 14 times its rated current at turn on. In practice, the surge current is typically <7 times the rated current, because of wiring resistance. e.g. a spotlight can draw a surge current of 37 A when it is first turned on, but draw only 5.9 A when it has been on for a while. The time constant of the surge current is typically <10 mS. Electric motors also draw high surge currents. Typical surge current for a trolling motor can be as much as 8 times its rated current, but seldom exceeds 65 A. The time constant of the surge current of a motor is typically <100 mS. A DC to AC inverter's surge current can be >100 A (and may only be limited by wiring resistance) with a typical time constant of <5 mS. Any of these surges can be experienced even when a DC load already exists on the battery. Therefore, the switch (or switches) must have a high surge current capability, even at high DC load currents.